David Meyer and James Lauroesch (Northwestern) used the DensePak fiber optic array on the 3.5-m WIYN Telescope at Kitt Peak to study the intervening interstellar gas in the direction of the globular cluster M15. Na I absorption seen along the line-of-sight to the various stars within M15 provides a rich map of the structure within the intervening interstellar medium. Meyer and Lauroesch's results imply either that there is significantly stronger fine-scale structure in the IMS than might have been expected or that there are significant variations in the Na/H ratio from point to point.
Recent studies of HI 21-cm absorption toward high velocity pulsars and extended extragalactic radio sources, as well as optical observations of the interstellar Na I D absorption toward globular cluster stars and binary stars, have revealed subparsec-scale structure in the diffuse interstellar medium. At the spatial scales sampled (~10 to 102 AU in pulsars, ~102 to 104 AU in binaries, and ~104 to 106 AU in globular clusters), the observations imply dense concentrations of atomic gas (nH ~103 cm -3) in otherwise diffuse sightlines. These dense concentrations are difficult to accommodate in the standard pressure equilibrium model of the interstellar medium, due to their large overpressures. Two approaches that might account for the observed column densities are a geometric model with lower density sheets or filaments aligned along the line of sight, or a turbulence-driven fractal model that produces structure at even small scales. Observations with both high spatial resolution and good spatial coverage are needed to understand the origin of these diffuse structures in the interstellar medium.
Two major interstellar components are known to exist between the Sun and M15—a local, low velocity (vLSR = +3 km s-1) cloud associated with the disk (LISM), and an intermediate velocity cloud (IVC) at vLSR = +68 km s-1. The distances to these components are estimated to be 500 pc and 1500 pc, respectively, based on their presence or absence in the spectra of stars nearby on the sky. The DensePak fiber array consists of 91 (3" diameter) fibers bonded into a 7×13 rectangle that covers 27"×43" of sky, with a center-to-center fiber spacing of 4". The bright, extended core of M15 provides a background source suitable for mapping the absorption-line structure of the intervening gas at high spatial resolution, in two dimensions, and with full sampling. At these distances, the 4" DensePak fiber spacing translates to spatial resolutions of 2000 and 6000 AU for the LISM and IVC, respectively. Column densities for the two clouds range from 2.3x1012 <= n(Na I) <= 8.5×1012 cm-2 (LISM) and from 5.0×1011 <= n(Na I) <= 8.0×1012 cm-2 (IVC).
The column density measurements are shown in the form of maps of the two clouds in the figures. The LISM and IVC gas toward M15 exhibits significant structure in terms of its physical conditions and/or H I column density down to arc second scales. The Na I data seem to rule out both a flat distribution on the 27"×43" scale of the DensePak array and a random distribution on the 4" scale of the individual fibers. Since Na I is not a dominant ion in HI clouds, the physical interpretation of the Na I structure can be ambiguous. However, the N(HI) column density inferred from the IVC N(Na I) column density is a factor of 10 higher than from 21-cm measurements. This implies either that there is significant clumpiness within the HI radio beam or that the N(Na I)/N(H) ratio in the IVC can be significantly higher than that typically observed in the diffuse ISM. In the case of the former, this result would have important implications for determining the metallicities of such halo clouds.